In thermodynamics, the cryoscopic constant, K_f, relates molality to freezing point depression (which is a colligative property). It is the ratio of the latter to the former:

${\displaystyle \Delta T_{f}=-i\cdot K_{f}\cdot b}$

• i is the van ‘t Hoff factor, the number of particles the solute splits into or forms when dissolved.
• b is the molality of the solution.

Through cryoscopy, a known constant can be used to calculate an unknown molar mass. The term "cryoscopy" comes from Greek and means "freezing measurement." Freezing point depression is a colligative property, so ΔT depends only on the number of solute particles dissolved, not the nature of those particles. Cryoscopy is related to ebullioscopy, which determines the same value from the ebullioscopic constant (of boiling point elevation).

The value of K_f, which depends on the nature of the solvent can be found out by the following equation:

${\displaystyle K_{f}={\frac {R\cdot M\cdot T_{f}^{2}}{\Delta _{\mathrm {fus} }H}}}$

• R is the ideal gas constant
• M is the molar mass of the solvent in kg mol⁻¹
• T_f is the freezing point of the pure solvent in kelvins
• Δ_fusH represents the molar enthalpy of fusion of the solvent in J mol⁻¹.

The K_f for water is 1.853 K kg mol⁻¹.